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L6924U
USB compatible battery charger system with integrated power switch for Li-Ion/Li-Polymer
Features
Fully integrated solution, with power MOSFET, reverse blocking diode, sense resistor, and thermal protection Charges single-cell Li-Ion batteries from selectable AC adapter or USB Input Programmable charge current up to 1 A in AC adapter mode Programmable charging current in USB mode for both high power and low power inputs 4.2 V output voltage with 1 % accuracy Linear or quasi-pulse operating mode Closed loop thermal control Programmable end-of-charge current Programmable charge timer (NTC) or (PTC) thermistor interface for battery temperature monitoring and protection Status outputs to drive LEDs or to interface with a host processor Small VFQFPN 16-leads package (3 mm x 3 mm) VFQFPN16

Applications

PDAs, GPS and MP3 players USB powered devices Cellular phones Digital still cameras Standalone chargers Wireless appliances
Table 1.
Device summary
Order codes L6924U VFQFPN16 L6924U013TR Tape and reel Package Packaging Tube
May 2008
Rev 1
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Contents
L6924U
Contents
1 2 Device description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1 3.2 Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4 5 6
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Operation description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1 6.2 Linear mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Quasi-pulse mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7
Applications information: charging process . . . . . . . . . . . . . . . . . . . . 17
7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 Charging process flow chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Pre-charge phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 AC or USB mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Fast charge phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 End-of-charge current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Recharge flow chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Recharge threshold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Maximum charging time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
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L6924U
Contents
8
Application information: monitoring and protection . . . . . . . . . . . . . . 23
8.1 8.2 8.3 8.4 NTC thermistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Battery absence detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Status pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
9
Additional applications information . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
9.1 9.2 Selecting input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
9.1.1 Selecting output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Layout guidelines and demonstration board . . . . . . . . . . . . . . . . . . . . . . . 30
10 11 12
Application idea: dual input management with AC priority . . . . . . . . 33 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
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Device description
L6924U
1
Device description
The L6924U is a fully monolithic battery charger that safely charges single-cell Li-Ion/Polymer battery from either USB power source or AC adapter. In USB mode, the L6924U supports both low power and high power mode. Alternatively the device can charge from AC wall adapter. The ideal solution for space-limited portable products is produced in BCD6 technology and integrates the power MOSFET, reverse blocking diode, sense resistor and thermal protection into a compact VFQFPN16 package. When an external voltage regulated adapter or USB port is used, the L6924U works in linear mode, and charges the battery in a constant current constant voltage (CC/CV) profile. Moreover, when a current-limited adapter is used, the device can operate in quasi-pulse mode, dramatically reducing the power dissipation. Regardless of the charging approach, a closed loop thermal control avoids device overheating. The device has an operating input voltage ranging from 2.5 V to 12 V and it allows the user to program many parameters, such as fast-charge current, end-ofcharge current threshold, and charge timer. The L6924U offers two open collector outputs for diagnostic purposes, which can be used to either drive two external LEDs or communicate with a host microcontroller. Finally, the L6924U also provides other features like gas gauge function, check for battery presence, and monitoring and protecting the battery from unsafe thermal conditions. Figure 1. Minimum size application board
Figure 2.
Basis application schematic
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L6924U
Pin description
2
Pin description
Figure 3. Pin connection (top view)
2.1
Pin description
Table 2.
Pin 1 2 3-4
Pin functions
I/O I I O Name VIN VINSNS Pin description Input pin of the power stage. Supply voltage pin of the signal circuitry. The operating input voltage range is from 2.5 V and 12 V, and the start-up threshold is 4 V.
ST2-ST1 Open-collector status pins. Maximum charging time program pin. It must be connected with a capacitor to GND to fix the maximum charging time, see Chapter 7.8: Maximum charging time on page 22. Ground pin. Shutdown pin. When connected to GND enables the device; when floating disables the device. Temperature monitor pin. It must be connected to a resistor divider including an NTC or PTC resistor. The charge process is disabled if the battery temperature (sensed through the NTC or PTC) is out of the programmable temperature window see Chapter 8.1: NTC thermistor on page 24.
5
I
TPRG
6 7
I
GND SD
8
I
TH
5/38
Pin description Table 2.
Pin
L6924U Pin functions (continued)
I/O Name Pin description Switch from high power USB (IUSB up to 500 mA) and low power USB (IUSB/5) in USB mode. A low level sets the L6924U in low power mode and a high level sets the L6924U in high power mode. When the AC mode is selected, the ISEL pin must be connected to ground or left floating. Output voltage sense pin. It senses the battery voltage to control the voltage regulation loop. Output pin. (connected to the battery) External reference voltage pin. (reference voltage is 1.8 V 2 %) Charge termination pin. A resistor connected from this pin to GND fixes the charge termination current threshold IENDTH: if ICHG < IENDTH, the charge process ends. The voltage across the resistor is proportional to the current delivered to the battery (gas gauge function). Select pin between AC adapter and USB port. A high level sets the L6924U in USB mode while a low level sets the L6924U in the AC adapter mode. When the AC adapter input is selected, the ISEL pin status does not affect the current set. Charge current program pin in USB mode: a resistor connected from this pin to ground fixes the fast charge current value (IUSB up to 500 mA) with an accuracy of 7 %. The USB high power/low power mode is selected with the ISEL pin. Charge current program pin in AC mode: a resistor connected from this pin to GND fixes the fast charge current value (IAC up to 1 A) with an accuracy of 7 %.
9
I
ISEL
10 11 12
I O O
VOSNS VOUT VREF
13
I/O
IEND
14
I
MODE
15
I
IUSB
16
I
IAC
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L6924U
Maximum ratings
3
Maximum ratings
Stressing the device above the rating listed in the "absolute maximum ratings" table may cause permanent damage to the device. These are stress ratings only and operation of the device at these or any other conditions above those indicated in the operating sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. Refer also to the STMicroelectronics sure program and other relevant quality documents.
3.1
Absolute maximum ratings
Table 3. Absolute maximum ratings
Parameter Input voltage Input voltage Output voltage Input voltage Output voltage Output current VREF, TH, IEND, IAC, IUSB, TPRG, GND All pins Maximum withstanding voltage range test condition: CDFAEC-Q100-002- "human body model" acceptance criteria: "normal performance' Value -0.3 to 16 -0.3 to VIN -0.3 to 5 -0.3 to 6 -0.3 to VIN 30 -0.3 to 4 Unit V V V V V mA V
Symbol VIN VINSNS, SD VOUT, VOSNS ISEL, MODE ST1, ST2
2
kV
3.2
Thermal data
Table 4.
Symbol RthJA TSTG TJ PTOT
Thermal data
Parameter Thermal resistance junction to ambient Storage temperature range Junction temperature range Power dissipation at T = 70 C
(1)
Value 75 -55 to 150 -40 to 125 0.67
Unit C/W C C W
1. Device mounted on demonstration board
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Electrical characteristics
L6924U
4
Table 5.
Symbol VIN (1) IIN (1)
Electrical characteristics
Electrical characteristics (TJ = 25 C, VIN = 5 V, unless otherwise specified)
Parameter Operating input voltage Start up threshold Supply current Charging mode (RPRG = 24 k) Shutdown mode (RPRG = 24 k) Shutdown mode (RPRG = 24 k) ISINK VOUT (1) IAC Current flowing from VOUT Battery regulated voltage Charge current with AC adapter input MODE at GND, RPRG = 24 k MODE at GND, RPRG = 12 k MODE at HIGH, ISEL at HIGH, RPRG-USB = 24 k MODE at HIGH, ISEL at LOW, RPRG-USB = 2 4 k MODE at GND, RAC = 24 k Stand by mode (RPRG = 24 k) (VIN = 2.5 V < VBATTERY) 4.16 450 905 450 86 41 41 7.6 2.9 REND = 3.3 k CTPRG = 10 nF R[IPRG] = 24 k CTPRG = 5.6n F RPRG = 24 k 12 4.2 490 975 490 96 49 49 9.6 3.0 16 3 10 2 0.4 Status on 10 1.3 0.4 1.3 0.4 1.8 60 Test condition Min 2.5 Typ Max 12 4.1 2.5 80 500 500 4.24 525 1045 525 mA 105 56 56 11.4 3.1 20 mA mA mA V mA hours % V V mA V V V V Unit V V mA A nA nA V mA mA
IUSB
Charge current with USB input
IPRE_AC
Pre-charge current with AC input
IPRE_USB
Pre-charge current with USB MODE at HIGH, ISEL at HIGH input (high power mode) RUSB = 24 k Pre-charge current with USB MODE at HIGH, ISEL at LOW input (low power mode) RUSB = 24 k Pre-charge voltage threshold Termination current
VPRETH IENDTH
TMAXCH (2) Maximum charging time TMAXCH (2) SDTH ST1,2 MODETH Maximum charging time accuracy Shutdown threshold high Shutdown threshold low Output status sink current MODE threshold high MODE threshold low ISEL threshold high ISELTH ISEL threshold low
8/38
L6924U Table 5.
Symbol RDS(on)
(3)
Electrical characteristics Electrical characteristics (continued) (TJ = 25 C, VIN = 5 V, unless otherwise specified)
Parameter Power MOSFET resistance NTC pin hot threshold voltage Test condition Charge current = 500 mA 10 40 Min Typ 280 12.5 50 Max 380 15 60 Unit m %VREF %VREF
TH NTC pin cold threshold voltage
1. TJ from -40 C to 125 C 2. Guaranteed by design 3. Device working in quasi pulse mode
9/38
Block diagram
L6924U
5
Block diagram
Figure 4. Block diagram
Logic I FAULT VIN
Logic I DETECT VOUT
POWER MOS VINS SD Logic ANALOG PRE. VDD Logic VREF Logic UVLO BODY CONTROL Gas Gauge VDD Mos Driver VOSNS IEND
VBG BG Logic
Charge Control
CA-VA-TA REG THERMAL CONTROL VREF
4.2V
ISEL MODE
IAC IUSB VREF
VPRE
LOGIC
VDD NTC/PTC MANAG.
TH
OSC
VDD
TPRG
ST2
ST1
GND
10/38
L6924U
Operation description
6
Operation description
The L6924U is a fully integrated battery charger that allows a very compact battery management system for space limited applications. It integrates in a small package, all the power elements: power MOSFET, reverse blocking diode and the sense resistor. It normally works as a linear charger when powered from an external voltage regulated adapter or USB port. However, thanks to its very low minimum input voltage (down to 2.5 V) the L6924U can also work as a quasi-pulse charger when powered from a current limited adapter. To work in this condition, it is enough to set the device's charging current higher than the adapter one (Chapter 6.2: Quasi-pulse mode on page 14). The advantage of the linear charging approach is that the device has a direct control of the charging current and so the designer needn't to rely on power source. However, the advantage of the quasi-pulse approach is that the power dissipated inside the portable equipment is dramatically reduced. With regard to the charging approach, the L6924U charges the battery in three phases:
Pre-charge constant current: in this phase (active when the battery is deeply discharged) the battery is charged with a low current (internally set to 10 % of the fastcharge current). Fast-charge constant current: in this phase the device charges the battery with the maximum current (IAC for AC adapter mode, IUSB for USB mode). Constant voltage: when the battery voltage is close to the selected output voltage, the device starts to reduce the current, until the charge termination is done. Programmable fast-charging current (IAC or IUSB) (Chapter 7.4 on page 19). Programmable end of charge current threshold (IENDTH) (Chapter 7.5 on page 20). Programmable end of charge timer (TMAXCH) (Chapter 7.8 on page 22).

The full flexibility is provided by:

If a PTC or NTC resistor is used, the device can monitor the battery temperature in order to protect the battery from operating in unsafe thermal conditions. Beside the good thermal behavior guaranteed by low thermal resistance of the package, additional safety is provided by the built-in temperature control loop. The IC monitors continuously its junction temperature. When the temperature reaches approximately 120 C, the thermal control loop starts working, and reduces the charging current, in order to keep the IC junction temperature at 120 C. Two open collector outputs are available for diagnostic purpose (status pins ST1 and ST2). They can be also used to drive external LEDs or to interface with a microcontroller. The voltage across the resistor connected between IEND and GND gives information about the actual charging current (working as a gas gauge), and it can be easily fed into a microcontroller ADC. Battery disconnection control is provided thanks to the differentiated sensing and forcing output pins. A small current is sunk and forced through VOUT. If VOSNS doesn't detect the battery, the IC goes into a standby mode. Figure 5 on page 12 shown the real charging profile of a Li-Ion battery, with a fast charge current of 450 mA (R1 or R2 = 26 k).
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Operation description Figure 5.
0.500 0.450 0.400 0.350 Ichg (A) 0.300 0.250 Ichg Vbatt
L6924U Li-Ion charging profile
4.500 4.000 3.500 3.000 2.500 2.000 Vbatt (V)
0.200 1.500 0.150 0.100 0.050 0.000 0 200 400 600 Charging time (sec) 800 1000 1.000 0.500 0.000 1200
6.1
Linear mode
When operating in linear mode, the device works in a way similar to a linear regulator with a constant current limit protection. It charges the battery in three phases:

Pre-charging current ("pre-charge" phase). Constant current ("fast-charge" phase). Constant voltage ("voltage regulation" phase).
VADP is the output voltage of the upstream AC-DC adapter that is, in turn, the input voltage of the L6924U. If the battery voltage is lower than the default pre-charge voltage (VPRETH), the pre-charge phase takes place. The battery is pre-charged with a low current , internally set to 10 % of the fast charge current. When the battery voltage goes higher than VPRETH, the battery is charged with the fast charge current (IUSB or IAC according to the selection of the MODE pin). Finally, when the battery voltage is close to the regulated output voltage (4.2 V), the voltage regulation phase takes place and the charging current is reduced. The charging process ends when the charging current reaches the programmed value (IENDTH) or when the charging timer expires. Figure 6 shows the different phases.
12/38
L6924U Figure 6. Typical charge curves in linear mode
Pre-Charge Phase
V ADP V OPRGTH
Operation description
Fast-Charge Phase Adapter Voltage
Voltage-Regulation Phase
End Charge
Battery Voltage
V PRETH
I CHG
Charge Current
I PRETH
Power dissipation
The worst case in power dissipation occurs when the device starts the fast-charge phase. In fact, the battery voltage is at its minimum value. In this case, there is the maximum difference between the adapter voltage and battery voltage, and the charge current is at its maximum value. The power dissipated is given by the following formula: Equation 1
PDIS = (VADP - VBAT ) x I CHG
The higher the adapter voltage is, the higher the power dissipated is. The maximum power dissipated depends on the thermal impedance of the device mounted on board.
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Operation description
L6924U
6.2
Quasi-pulse mode
The quasi-pulse mode can be used when the system can rely on the current limit of the upstream adapter to charge the battery. In this case, the fast charge current must be set higher than the current limit of the adapter. In this mode, the L6924U charges the battery with the same three phases as in Linear Mode, but the power dissipation is greatly reduced as shown in Figure 7. Figure 7. Typical charge curves in quasi pulse mode
Pre-Charge Phase
V ADP V O PRGTH
Fast-Charge Phase
Voltage Regulation Phase Adapter Voltage Battery Voltage
End Charge
V PRETH
Ilim x Rdson
I CHG I LIM
Charge Current
I PRETH
Power dissipation
The big difference is due to the fact that the charge current is higher than the current limit of the adapter. During the fast-charge phase, the output voltage of the adapter drops and goes down to the battery voltage plus the voltage drop across the power MOSFET of the charger, as shown in the following equation: Equation 2
VIN = VADP = VBAT + VMOS
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L6924U
Operation description
Where VMOS is given by: Equation 3
V
MOS
= R DS ( ON ) x I LIM
Where, ILIM = current limit of the wall adapter, and RDS(on) = resistance of the power MOSFET. The difference between the programmed charge current and the adapter limit should be high enough to minimize the RDS(on) value (and the power dissipation). This makes the control loop completely unbalanced and the power element is fully turned on. Figure 8 shows the RDS(on) values for different output voltage and charging currents for an adapter current limit of 500 mA. Figure 8. RDS(on) curves vs charging current and output voltage
15/38
Operation description
L6924U
Neglecting the voltage drop across the charger (VMOS) when the device operates in this condition, its input voltage is equal to the battery one, and so a very low operating input voltage (down to 2.5 V) is required. The power dissipated by the device during this phase is: Equation 4
PCH = RDS ( on ) x I LIM
2
When the battery voltage approaches the final value, the charger gets back the control of the current, reducing it. Due to this, the upstream adapter exits the current limit condition and its output goes up to the regulated voltage VADP. This is the worst case in power dissipation: Equation 5
PDIS = (VADP - VBAT ) x I LIM
In conclusion, the advantage of the linear charging approach is that the designer has the direct control of the charge current, and consequently the application can be very simple. The drawback is the high power dissipation. The advantage of the quasi-pulse charging method is that the power dissipated is dramatically reduced. The drawback is that a dedicated upstream adapter is required.
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L6924U
Applications information: charging process
7
7.1
Applications information: charging process
Charging process flow chart
Figure 9. Charging process flow chart
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Applications information: charging process
L6924U
7.2
Pre-charge phase
The L6924U allows pre-charging the battery with a low current when the battery is deeply discharged. The battery is considered deeply discharged when its voltage is lower than a threshold (VPRETH), internally set to 3 V. During pre-charge phase, the current (IPRECH) has a default value equal to 10 % of the fastcharge current. A safety timer is also present. If the battery voltage does not rise over VPRETH within this time, a fault is given (Chapter 7.8: Maximum charging time on page 22). If at the beginning of the charge process, the battery voltage is higher than the VPRETH, the pre-charge phase is skipped.
7.3
AC or USB mode
The L6924U can charge batteries from both AC adapter and USB inputs. The choice of the power supply is done by driving the MODE pin. A low level sets the L6924U in AC mode. The fast charge current is determined by the resistor connected to the IAC pin (Chapter 7.4: Fast charge phase), regardless the resistor connected to IUSB. The same way, a high level at the MODE pin sets the L6924U in USB mode. The fast charge current is determined by the resistor connected to the IUSB pin (Chapter 7.4: Fast charge phase), regardless the resistor connected to IAC. Figure 10. MODE pin selection
Sets the fast charge current IUSB IUSB Sets the fast charge current
L6924U
IAC
L6924U
IAC
MODE
RAC
RUSB VIN
MODE
RAC
RUSB
AC adapter mode
USB mode
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L6924U
Applications information: charging process
7.4
Fast charge phase
When the battery voltage reaches the pre-charge voltage threshold (VPRETH), the L6924U enters the fast-charge phase. In this phase the device charges the battery with a constant current, whose value can be set by external resistors connected to IAC pin (AC adapter mode selected) or to IUSB pin (USB mode) with an accuracy of 7 %. In AC adapter mode (MODE pin low), the resistor RAC can be calculated as: Equation 6
V R AC = BG K PRG I AC
Where VBG is the internal reference equal to 1.23 V, whereas KPRG is a constant equal to 9500. Figure 11. IAC pin connection
In USB mode (MODE pin high), the resistor RUSB can be selected as: Equation 7
V RUSB = BG KPRG I USB
Where VBG and KPRG have the same meaning and value above mentioned. The charge current in USB mode depends on RUSB as well as the state of the ISEL pin. When this pin is high, the "high-power" USB mode is selected and the charge current is determined by the equation 7. The charge current in USB mode should be set in accordance with the typical USB current capability (up to 500 mA). If ISEL is low, the "low-power" USB mode is selected and the charge current is a fifth of the high-power USB mode charge current (up to 100 mA) During low power USB mode operation, since the charge current is reduced by one fifth, the maximum charging time is proportionally increased (Section 7.8: Maximum charging time).
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Applications information: charging process Figure 12. IUSB pin connection
L6924U
Regardless of the operation mode (AC adapter or USB), during the fast-charge phase the battery voltage increases until it reaches the programmed output voltage (4.2 V). A safety timer is also present. If the Fast-charge phase does not finish within the programmed time (see Chapter 7.8: Maximum charging time on page 22), a fault is given.
7.5
End-of-charge current
When the charge voltage approaches the battery regulated voltage (internally set to 4.2 V), the voltage regulation phase takes place. The charge current starts to decrease until it goes below a programmable termination current, IENDTH. This current can be selected by an external resistor connected between the IEND pin and GND Figure 13, whose value can be calculated as:
Equation 8
KEND REND = VMIN x I ENDTH
Figure 13. IEND pin connection
E
Where KEND is 1050; and VMIN is 50 mV. When the charge current goes below IENDTH, after a deglitch time, the status pins notify the end of charge and the charge process ends.
This de-glitch time is expressed as:
Equation 9
TDEGLITCH =
TMAXCH 220
where TMAXCH is the maximum charging time. (Chapter 7.8 on page 22)
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L6924U
Applications information: charging process
IEND pin is also used to monitor the charge current, because the current injected in REND is proportional to the charge current. The voltage across REND can be used by a microcontroller to check the charge status like a gas gauge.
7.6
Recharge flow chart
Figure 14. Recharge flow chart
7.7
Recharge threshold
When, from an end-of-charge condition, the battery voltage goes below the recharging threshold (VRCH), the device goes back in charging state. The value of the recharge threshold is 4.05 V.
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Applications information: charging process
L6924U
7.8
Maximum charging time
To avoid the charging of a dead battery for a long time, the L6924U has the possibility can be set a maximum charging time starting from the beginning of the fast-charge phase. This timer can be set with a capacitor, connected between the TPRG pin and GND. The CTPRG is the external capacitor (in nF) and is given by the following formula:
Equation 10
C TPRG
T MAXCH V BG x R PRG KT = V REF
x 10 9
E
Note:
The maximum recommended CTPRG value must be less than 50 nF. Figure 15. TPRG pin connection
Where, RPRG = resistor which sets the current (RUSB or RAC) VREF = 1.8 V, KT = 279 x 105, VBG = 1.23 V, and TMAXCH is the charging time given in seconds. If the battery does not reach the end-of-charge condition before the time expires, a fault is issued. Also during the pre-charge phase there is a safety timer, given by:
Equation 11
1 TMAXPRECH = x TMAXCH 8
If this timer expires and the battery voltage is still lower than VPRETH, a fault signal is generated, and the charge process finishes.
Note: When the device is charged in low power USB mode, in order to take into account the reduced charge current, the maximum charging time is proportionally increased (five times the maximum charging time calculated with RUSB)
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L6924U
Application information: monitoring and protection
8
Application information: monitoring and protection
The L6924U uses a VFQFPN 3 mm x 3 mm 16-pin package with an exposed pad that allows the user to have a compact application and good thermal behavior at the same time. The L6924U has a low thermal resistance because of the exposed pad (approximately 75 C/W, depending on the board characteristics). Moreover, a built-in thermal protection feature prevents the L6924U from having thermal issues typically present in a linear charger. Thermal control is implemented with a thermal loop that reduces the charge current automatically when the junction temperature reaches approximately 120 C. This avoids further temperature rise and keeps the junction temperature constant. This simplifies the thermal design of the application as well as protects the device against over-temperature damage. The figure above shows how the thermal loop acts (with the dotted lines), when the junction temperature reaches 120 C.
Figure 16. Power dissipation both linear and quasi pulse mode with thermal loop
23/38
Application information: monitoring and protection
L6924U
8.1
NTC thermistor
The device allows designers to monitor the battery temperature by measuring the voltage across an NTC or PTC resistor. Li-Ion batteries have a narrow range of operating temperature, usually from 0 C to 50 C. This window is programmable by an external divider which is comprised of an NTC thermistor connected to GND and a resistor connected to VREF. When the voltage on the TH pin exceeds the minimum or maximum voltage threshold (internal window comparator), the device stops the charge process, and indicates a fault condition through the status pin. When the voltage (and thus, the temperature), returns to the window range, the device restarts the charging process. Moreover, there is a hysteresis for both the upper and lower thresholds, as shown in Figure 18.
Figure 17. Battery temperature control flow chart
Note:
TBAT = OK when the battery temperature between 0 C and 50 C Figure 18. Voltage window with hysteresis on TH
VMINTH VMINTH_HYS
900mV
780mV
Voltage Variation on TH pin
Charge disable Charge enable
VMAXTH_HYS VMAXTH
248mV
225mV
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L6924U
Application information: monitoring and protection
Figure 19.
Pin connection
When the TH pin voltage rises and exceeds the VMINTH = 50 % of VREF (900 mV typ), the L6924U stops the charge, and indicates a fault by the status pins. The device re-starts to charge the battery, only when the voltage at the TH pin goes under VMINTH_HYS = 780 mV (typ). For what concerns the high temperature limit, when the TH pin voltage falls under the VMAXTH = 12.5 % of VREF (225 mV Typ.), the L6924U stops the charge until the TH pin voltage rises at the VMAXTH_HYS = 248 mV (Typ.). When the battery is at the low temperature limit, the TH pin voltage is 900 mV. The correct resistance ratio to set the low temperature limit at 0 C can be found with the following formula:
Equation 12
VMINTH = VREF x
RNTC 0C RUP + RNTC 0C
Where RUP is the pull-up resistor, VREF is equal to 1.8 V, and RNTC0C is the value of the NTC at 0 C. Since at the low temperature limit VMINTH = 900 mV:
Equation 13
0.9 = 1.8 x
It follows that:
Equation 14
RNTC 0C RUP + RNTC 0C
RNTC 0C = RUP
Similarly, when the battery is at the high temperature limit, the TH pin voltage is 225 mV. The correct resistance ratio to set the high temperature limit at 50 C can be found with the following formula:
Equation 15
VMAXTH = VREF x
RNTC 50C RUP + RNTC 50C
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Application information: monitoring and protection
L6924U
Where RNTC50C is the value of the NTC at 50 C. Considering VMAXTH = 225 mV it follows that:
Equation 16
0.225 = 1.8 x
Consequently:
Equation 17
RNTC 50C RUP + RNTC 50C
RNTC 50C =
Based on equations 14 and 17, it derives that:
Equation 18
RUP 7
RNTC 0C =7 RNTC 50C
The temperature hysteresis can be estimated by the formula:
Equation 19
THYS =
VTH - VTH _ HYS VTH x NTCT
Where VTH is the pin voltage threshold on the rising edge, VTH_HYS is the pin voltage threshold on the falling edge, and NTCT (- %/C) is the negative temperature coefficient of the NTC at temperature (T) expressed in % resistance change per C. For NTCT values, see the characteristics of the NTC manufacturers (e.g. the 2322615 series by VISHAY). At the low temperature, the hysteresis is approximately:
Equation 20
THYS 0C =
900mV - 780mV 900mV x NTC 0C
Obviously at the high temperature hysteresis is:
Equation 21
THYS 50C =
225mV - 248mV 225mV x NTC 50C
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L6924U
Application information: monitoring and protection
Considering typical values for NTC0C and NTC50C, the hysteresis is:
Equation 22
THYS 0C =
And:
Equation 23
900mV - 780mV 2.5o C 900mV x 0.051
THYS 50C =
225mV - 248mV -2.5o C 225mV x 0.039
If a PTC connected to GND is used, the selection is the same as above, the only difference is when the battery temperature increases, the voltage on the TH pin increases, and vice versa. For applications that do not need a monitor of the battery temperature, the NTC can be replaced with a simple resistor whose value is one half of the pull-up resistor RUP. In this case, the voltage at the TH pin is always inside the voltage window, and the charge is always enabled.
8.2
Battery absence detection
This feature provides a battery absent detection scheme to detect the removal or the insertion of the battery. If the battery is removed, the charge current falls below the IENDTH. At the end of de-glitch time, a detection current IDETECT, equal to 1 mA, is sunk from the output for a time of TDETECT. The device checks the voltage at the output. If it is below the VPRETH, a current equal to IDETECT is injected in the output capacitor for a TDETECT, and it is checked to see if the voltage on the output goes higher than VRCH (4.05 V). If the battery voltage changes from VPRETH to VRCH and vice versa in a TDETECT time, it means that no battery is connected to the charger. The TDETECT is expressed by:
Equation 24
TDETECT =
TMAXCH 54x 103
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Application information: monitoring and protection
L6924U
Figure 20. Battery absence detection flow chart
DETECT LOW ABSENT = a ISINK is sunk for a TDET from the battery DETECT HIGH ABSENT = a IINJ is injected for a TDET in the battery TDET = 100ms (Typ.) ISINK = IINJ = 1mA (Typ.)
BATTERY ABSENT
Detect Low Absent
VBAT > VPRETH
YES
FAST CHARGE
NO
Detect High Absent
YES
VBAT > VRCH
NO
PRE CHARGE
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L6924U
Application information: monitoring and protection
8.3
Status pins
To indicate various charger status conditions, there are two open-collector output pins, ST1 and ST2. These status pins can be used either to drive status LEDs, connected with an external power source, by a resistor, or to communicate to a host processor.
Figure 21. ST1 and ST2 connection with LEDs or microcontroller
Table 6.
Status LEDs Indications
Description When the device is in pre-charge or fastcharge status When the charging current goes below the IENDTH When the input voltage goes under VBAT + 50 mV When the voltage on the TH pin is out of the programmable window, in accordance with the NTC or PTC thermistor When the battery pack is removed When TMAXCH or TMAXPRECH expires ST1 ON OFF OFF ST2 OFF ON OFF
Charge condition Charge in progress Charge done Stand by mode
Bad battery temperature Battery absent Over time
ON ON ON
ON ON ON
8.4
Shutdown
The L6924U has a shutdown pin; when the pin is connected to GND, the device is operating. When the pin is left floating, the device enters the shutdown mode, the consumption from the input is dramatically reduced to 60 A (typ.). In this condition, VREF is turned OFF.
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Additional applications information
L6924U
9
9.1
Additional applications information
Selecting input capacitor
In most applications, a 1 F ceramic capacitor, placed close to the VIN and VINSN pins can be used to filter the high frequency noise.
9.1.1
Selecting output capacitor
Typically, a 4.7 F ceramic capacitor placed close to the VOUT and VOUTSN pin is enough to keep voltage control loop stable. This ensures proper operation of battery absent detection in removable battery pack applications.
9.2
Layout guidelines and demonstration board
The thermal loop keeps the device at a constant temperature of approximately 120 C which in turn, reduces ICHG. However, in order to maximize the current capability, it is important to ensure a good thermal path. Therefore, the exposed pad must be properly soldered to the board and connected to the other layer through thermal vias. The recommended copper thickness of the layers is 70 m or more. The exposed pad must be electrically connected to GND. Figure 22 shows the thermal image of the board with the power dissipation of 1 W. In this instance, the temperature of the case is 89 C, but the junction temperature of the device is given by the following formula:
Equation 25
TJ = RTHJ - A x PDISS + TAMB
Where the RthJA of the device mounted on board is 75 C/W, the power dissipated is 1 W, and the ambient temperature is 25 C. In this case the junction temperature is:
Equation 26
TJ = 75 x1 + 25 = 100o C
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L6924U
Additional applications information
Figure 22. Thermal image of the demonstration board
The VOSNS pin can be used as a remote sense; so, it should be connected as closely as possible to the battery. The demonstration board layout and schematic are shown in Figure 24 and Figure 25.
Figure 23. Demonstration board layout, top side
Figure 24. Demonstration board layout, bottom side
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Additional applications information Figure 25. Demonstration board schematic
L6924U
Table 7.
Name R1 R2 R3 R4 R5 R6 RT1 C1 C2 C3 C4 D1 D2 J1 J2 J3 J4 J5
Demonstration board components description
Value 24 k 24 k 3.3 k 1 k 1 k 1 k 470 1 F 4.7 F 10 nF 1 nF Description AC mode fast-charge current resistor. Used to set the charging current in AC mode. USB mode fast-charge current resistor. Used to set the charging current in USB mode. End of Charge current resistor. Used to set the termination current and, as a "gas gauge" when measuring the voltage across on it. Pull up resistor. Connected between VREF and TH pin. Pull up resistor. To be used when the ST1 is connected with a LED. Pull up resistor. To be used when the ST2 is connected with a LED. If a NTC is not used, a half value of R4 must be mounted to keep the TH voltage in the correct window. Input capacitor. Output capacitor. TMAX capacitor. Used to set the maximum charging time. VREF filter capacitor.
GREEN ST1 LED. RED ST2 LED. ST2 jumper. Using to select the LED or the external microcontroller. ST1 jumper. Using to select the LED or the external microcontroller. SD jumper. If open, the device is in SD mode; when closed, the device starts to work. Low power/ high power USB mode selection jumper. AC/USB mode selection jumper.
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L6924U
Application idea: dual input management with AC priority
10
Application idea: dual input management with AC priority
In some applications both AC adapter and USB power source may be available.
Figure 1 shows a possible schematic which provides the possibility to manage two power sources (AC/USB) and gives the priority to AC adapter in case both sources are available at the same time.
For simplicity, only the relevant parts of L6924U for this application have been indicated. If only the AC adapter is available, since the gates of Q1 and Q2 are connected to AC, both MOSFETs are off. The AC adapter voltage is provided to the pin VIN through the diode D1. The voltage of the pin VIN is: VIN = VAC - Vdiode A correct choice of this diode is important to limit Vdiode and keeping VIN as close as possible to AC. In this condition the pin MODE is low. This sets the L6924U in AC mode and the battery is charged with the current programmed by RAC. When only the USB power source is available, both Q1 and Q2 switch on and the pin VIN is connected to USB. The pin MODE is connected to the drains of Q1 and Q2 and is high. Therefore USB mode for L6924U is selected and the battery is charged with a current in accordance with the resistor connected to the pin IUSB (RUSB). The voltage of the pin VIN is given by:
VIN = VUSB - (RDSon _ Q1 + RDSon _ Q2 ) IUSB
The voltage drop across the MOSFETs must be kept as low as possible to avoid reducing too much the voltage of the pin VIN. When both sources are present, this circuit gives the priority to the AC adapter. In fact, for VAC 5 V, surely both Q1 and Q2 are off and VIN is connected to the AC adapter through D1. The pin MODE is kept low and L6924U is set to AC mode. The use of two P-channel MOSFETs connected as shown in Figure 1 is particularly useful in this case because they remove any path between the two power sources.
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Application idea: dual input management with AC priority Figure 26. Dual input management
AC Q1 USB D1 Q2 VIN VOUT
Li-Ion battery
L6924U
L6924U
IUSB IAC RAC RUSB
RG RM
MODE
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L6924U
Package mechanical data
11
Package mechanical data
In order to meet environmental requirements, ST offers these devices in ECOPACK(R) packages. These packages have a lead-free second level interconnect . The category of second Level Interconnect is marked on the package and on the inner box label, in compliance with JEDEC Standard JESD97. The maximum ratings related to soldering conditions are also marked on the inner box label. ECOPACK is an ST trademark. ECOPACK specifications are available at: www.st.com.
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Package mechanical data
L6924U
Table 8.
Dim.
VFQFPN16 (3 mm x 3 mm) mechanical data
mm. Min Typ 0.900 0.020 0.650 0.250 0.180 2.875 0.250 2.875 0.250 0.450 0.300 0.230 3.000 0.700 3.000 0.700 0.500 0.400 0.300 3.125 1.250 3.125 1.250 0.550 0.500 0.080 0.007 0.113 0.009 0.113 0.009 0.017 0.011 Max 1.000 0.050 1.000 Min 0.031 inch Typ 0.035 0.001 0.025 0.010 0.010 0.120 0.027 0.118 0.027 0.019 0.015 0.012 0.123 0.050 0.123 0.049 0.021 0.019 0.003 Max 0.039 0.002 0.039
A A1 A2 A3 b D D2 E E2 e L ddd
0.800
Figure 27. Package dimensions
E
A K A1
E2
e D b K r D2
A3
L
This drawing is not to scale
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L6924U
Revision history
12
Revision history
Table 9.
Date 20-May-2008
Document revision history
Revision 1 First release Changes
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L6924U
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